growled while playing

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Sophie is almost 4 months old. She loves to bring her toys to us so we can play with her. While playing fetch with her tonight, my 2 year old son tried to grab her rope toy from her. Every time he tried to grab it from her, she would growl at him but she wouldn't if my husband or myself grabbed it from her. My husband seemed to think she was playing with him. I beg to differ. This is our first Akita and our first dog as a family so I am totally unsure as to whether this is normal. This is the first time she's ever growled. If this is not normal, how do we correct it?